Yemeni government forces clashed with separatists in the southern port city of Aden on Sunday.
An Associated Press journalist heard heavy gunfire in the Khor Maksar district, while thick black smoke rose in the sky after an airstrike.
Witnesses, speaking on condition of anonymity because they feared reprisal, said residents were fleeing the area. Schools and universities were shut down and students were told to stay home.
President Abed Rabbo Mansour Hadi's forces, which have been at war with Shiite rebels known as Houthis since March 2015, deployed across the city to prevent protests by southerners who want to bring down his government and separate from the Houthi-dominated north.
The protests are being led by Aidarous al-Zubaidi, a former Aden governor backed by the United Arab Emirates. The UAE is a key member of the Saudi-led coalition allied with Hadi's government, but has had tense relations with the president.
The UAE has trained and armed forces, known as the Security Belt, who do not answer to Hadi's government.
On Saturday, the coalition issued a statement calling for "self-restraint" and described the protests as "popular demands to fix government flaws."
Aden, the seat of Hadi's government, has seen a sharp deterioration in services amid accusations of corruption and wasting public funds. Hadi supporters blame the coalition, saying it has failed to fulfill promises to rebuild Aden while barring the president, his sons and top commanders from returning to the country.
The tug-of-war between UAE and Hadi has fractured the coalition and contributed to the nearly three-year stalemate with the Houthis, who control the capital, Sanaa, and much of northern Yemen. The war has killed more than 10,000 people and displaced 2 million. The U.N. says it is the world's worst humanitarian crisis.
Northern and Southern Yemen were unified in 1990. Southerners have long advocated greater autonomy or outright secession, complaining of domination by the north. But not all separatists have joined forces with al-Zubaidi, and many fear a return of the civil war that raged in South Yemen from 1986 until unification.
"We won't allow a repeat of the painful past in Aden and we will not allow disturbances," Prime Minister Ahmed Obaid Bin Daghar warned on his Twitter. "The government won't be the reason behind it."
